1. Field of the Invention
The present invention pertains to a method for correcting errors that occur during the use of Crossfeed Radar (CFR) equipment that operates in accordance with a crossfeed principle and which receives echo signals from flying objects. In particular, the present invention is used in connection with echo signals that are emitted by a low-flying object, such as an aircraft, and are reflected from a surface of the earth.
2. Discussion of Background and Relevant Information
Monopulse tracking radar equipment is normally used to track airborne objects, such as an aircraft. Such a system purposely generates azimuth and elevation error signals which, when transmitted in open space, cancel out azimuth and elevation errors if the transmitting antenna, or boresight axis, is directly aligned with a desired target. An antenna guidance system, using two servo circuits, is typically employed for tracking a target. One servo circuit controls the elevation of the antenna, while the second servo circuit controls the azimuth adjustment of the antenna.
Unfortunately, an elevation angle error occurs when a transmitted signal "hits", for example, a body of water, and is echoed back to the antenna. This results in a non-zero elevation angle indication that can not be eliminated, even if the axis of the antenna is aligned directly with the target. Thus, in addition to receiving the desired echo signal from the tracked target, a second, undesired, target echo signal, reflected from the surface of the water (or ground), is received. The radar system cannot discriminate between the first, desired echo signal and the second, undesired echo signal with regard to the distance or bearing. Thus, the superimposition of these two echo signals results in an erroneous indication of the elevation of the target. The result is that during the guidance tracking phase, the elevation servo circuit orients the antenna axis away from the target.
Additional bearing error signals have been utilized in a so-called crossfeed method in an attempt to overcome the above-discussed deficiency of the monopulse tracking system. If the right combination of signals is used, a zero position signal on the antenna axis will be generated. Such a method is described in, for example, Swiss Letters Patent Nos. 592,887 and 629,898. According to these patents, a monopulse antenna with, for example, four antenna horns a, b, c, d is used. The antenna horns receive an interference signal, consisting of a direct echo signal and a reflected echo signal. From the echo signals received by the antenna horns a, b, c and d, a summing/differentiation circuit located downstream obtains the following signals which depend upon the alignment of the antenna with respect to the target and to the reflecting surface: EQU F.sub.S a+b+c+d, (1)
where F.sub.S comprises a summing function containing the distance information EQU F.sub.dA =d-b, (2)
where F.sub.dA comprises a first differentiation function containing lateral or azimuth bearing information EQU F.sub.dE =a-c, (3)
where F.sub.dE comprises a second differentiation function that is distorted by reflection effects, and which contains an elevation angle or elevation information and reflection effect information EQU F.sub.dK =(a+c)-(b+d), (4)
where F.sub.dK comprises a cross-term function that contains both bearing data and reflection effect information.
These functions are processed in accordance with the radar system, in reference to the summing function F.sub.S in a discriminator that is located downstream of the summing/differentiation circuit. This results in the production of three signals:
Azimuth angle signal F.sub.A =F.sub.dA /F.sub.S ; PA1 Elevation angle signal F.sub.E =F.sub.dE /F.sub.S ; and PA1 Crossterm signal F.sub.K =F.sub.dK /F.sub.S.
In adjacent quadrature channels, the in-phase portions EAP, EEP and EKP of the bearing error functions F.sub.A, F.sub.E, and F.sub.K and the quadrature portions EEQ and EKQ which are displaced by 90 degrees with respect to the corresponding in-phase portions EEP and EKP, are obtained. The summing function F.sub.S, or a signal corresponding to it, is valid as a reference for all bearing error signals.
A corrected elevation angle error signal F.sub.Ekorr is obtained as a function of the angle between the target and the antenna axis w.sub.dE from the function: EQU F.sub.Ekorr (w.sub.dE)=EEP-(EEQ/EKQ).multidot.EKP+(EEQ/EKQ).multidot.F.sub.K (w.sub.dE) EQU or EQU F.sub.Ekorr (w.sub.dE)=EEP+FD1+FD2
In contrast to correction term FD1 itself, remainder correction term FD2 is a function of angle w.sub.dE and is iteratively determined, or is disregarded. Error correction in accordance with the described method operates satisfactorily as long as the reflection echo signal is reflected to the antenna from only one place on the surface of the earth. However, multiple, or diffuse reflections, directed towards the receiver antenna often occur, which cannot be corrected at all, or can only be partially corrected by the above-described method.